Implement coreutils-ext package with additional multiplatform programs

[certik]

That way, they could also be installed separately by users of bash/zsh and don't hinder compatibility concerns.

Candidates:

• open: Implement open #109
• dl (download): Implement a way to download files from the internet #110
• tar, gz, bz2, unzip: Add tar, gz, bz2 and unzip #131
• ldd: Add multiplatform "ldd" equivalent #140
• kill, ps: Process control: kill, ps, etc. #142
• time: Implement time #145
• printenv: Support printenv #137

More utilities to consider:

• pushd/popd
• chmod
• chown
• diff
• df
• du
• find
• grep
• xargs
• ping
• more / less
• sleep
• cut

Some other commonly used tools:

• shutdown / reboot
• split
• ssh / ssh-keygen / scp
• vi
• rsync

Full list of the most common commands: https://www.busybox.net/downloads/BusyBox.html